1. Field of the Invention
The present invention relates to communications systems of the type that characterize received signals according to historical state transitions and particularly to the generation of reliability information used by such systems in characterizing received signals.
2. Discussion of the Related Art
In digital communication systems, the frequency-selective channels often introduce intersymbol interference (ISI) on the transmitted symbols. To mitigate the resulting distortion effects and channel impairment, digital communication systems often implement digital demodulators designed to compensate for the effects of intersymbol interference. One of the popular demodulators for digital communication systems includes a Viterbi algorithm (VA) equalizer that provides outputs to a de-interleaver and a convolutional decoder. The Viterbi algorithm is suitable for equalization and decoder applications and is described in “Viterbi Algorithm,” by G. David Forney Jr., Proceedings of IEEE, Vol. 61, No. 3, pp. 268–278 (1973). Some applications of the Viterbi algorithm with soft outputs are described in “A Viterbi Algorithm with Soft-Decision Outputs and its Applications,” by J. Hagenauer and P. Hoeher, Proceedings of the GLOBECOM'89, No. 47.1, pp. 1680–1686 (1989). In the paper by Hagenauer, et al., the advantages of providing soft outputs of reliability information to the decoder for use in decoding symbols are outlined. This method uses a large memory space for storing accumulated candidate node metrics and extra trace-back procedures.
The Viterbi algorithm equalizer is an efficient receiver that uses maximum likelihood sequence estimation (MLSE). Viterbi algorithm equalizers achieve 1–4 dB gain if the equalizer can deliver soft decisions to the de-interleaver and convolutional decoder rather than hard decisions. Therefore, soft-output Viterbi algorithm (SOVA) equalizers and one-step soft output Viterbi algorithm equalizers presently prevail in receiver designs. Most soft output Viterbi algorithm equalizers store the soft values of all states during processing and while performing the trace back procedure over all possible prior states to decode the received symbol sequence. U.S. Pat. No. 5,119,400 proposes a Viterbi-like equalizer that does not store the soft values of all states during processing and does not utilize a trace back procedure. Better reliability information can be obtained using the system described in the patent.
U.S. Pat. No. 5,119,400 discusses a reliability information strategy that identifies all possible paths whose 2L last state transitions correspond to a binary value “1” and all possible paths whose L last state transitions correspond to a binary “0.” The path having the larger probability function is selected, the binary value assigned to the larger of the two selected values is taken as the estimate ak−L and the reliability information is obtained from the two selected probability functions. By using this method, neither a large memory space nor trace back procedures are used by the Viterbi-like equalizer and a significant amount of memory and processing time can be saved.